This invention relates to ceramic articles, such as fibers, and to a process for producing the same.
"High-technology ceramics"--also known as high-performance ceramics, engineering ceramics, technical ceramics, advanced ceramics, and structural ceramics, and distinct from such conventional ceramics as pottery, dinnerware, cement, building bricks, roof tiles, and window glass--have excellent mechanical properties under heavy stress, outstanding electrical and optical properties, and exceptional resistance to high temperatures and corrosive environments.
High-technology ceramic articles having a core and a sheath are known in the art. U.S. Pat. No. 4,125,406 (Sowman) discloses ceramic fibers having at least two distinct zones of composition and properties. One zone was the core of the fiber and the second zone was a sheath or skin which is formed on the fiber during firing. U.S. Pat. No. 3,778,132 (Pinnow et al) discloses a transmission line structure having a core section made of pure amorphous silica and an encompassing section made of boron oxide (B.sub.2 O.sub.3) admixed with silica which may be applied as a single layer cladding. U.S. Pat. No. 3,779,784 (Emslie) discloses fibers having a leachable core of alkali metal hexatitanate encapsulated by a shell of rutile (TiO.sub.2); the fibrous material is particularly useful in reinforcing plastics.
U.S. Pat. No. 3,881,944 (Beall et al) discloses glass-ceramic articles demonstrating very high refractoriness, i.e., having a use temperature of at least 1,200.degree. C. and a low coefficient of thermal expansion, i.e., less than 40.times.10.sup.-7 /.degree.C. over the temperature range of 25.degree.-900.degree. C., which, when subjected to certain leaching treatments, will exhibit at least a surface layer manifesting a high and relatively uniform porosity with the vast majority of the pores having diameters ranging between about 0.1-40 micrometers to provide a surface area of about 1-100 meters.sup.2 /gram. Glass-ceramic bodies wherein a principal crystal phase is mullite, celsian, beta-spodumene, or cordierite can, in certain compositional areas, be leached to yield the above-described products. Hot dilute aqueous solutions of mineral acids such as HCl, HNO.sub.3, H.sub.2 SO.sub.4, H.sub.3 PO.sub.4, etc., act to preferentially leach the celsian and cordierite crystals to yield porous bodies of high surface area but with extremely fine pores. Dilute aqueous solutions at room temperature (R.T.) of NaOH, HF, or NH.sub.4 F:HF preferentially leach the beta-spodumene solid solution crystals to produce bodies with larger pores but also with much lower surface areas.
Although the foregoing patents describe products having useful properties, none of them describe an article with a capability of being filled or infiltrated with certain substances in a controlled geometry and in controlled amounts to render the article useful for applications previously considered unattainable. The article described in U.S. Pat. No. 3,778,132 is a glass article which is not designed for being infiltrated with substances. The article described in U.S. Pat. No. 3,779,784 would not be expected to be useful for infiltration applications. U.S. Pat. No. 4,125,406 describes an article which exhibits many desirable properties, but this patent does not describe infiltration of that article with high levels of substances. U.S. Pat. No. 3,881,944 does not in actuality describe a product having a core and a sheath; the patent does not indicate that the surface layer described therein has the ability to be filled with a high level of material.